Silicon carbide, a crystalline compound of metallic silicon and nonmetallic carbon, has long been known for its hardness, its strength, and its excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties, and maintains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction bonding, chemical vapor deposition, hot pressing and pressureless sintering (initially forming the article and subsequently sintering). Examples of these methods are described in U.S. Pat. Nos. 3,853,566; 3,852,099; 3,954,483; and 3,960,577. The high density silicon carbide bodies so produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange units, pumps, and other equipment or tools that are exposed to severe wear and/or operation under high temperature conditions. The present invention relates methods of producing silicon carbide articles that have high-density and high-strength characteristics.
In order to obtain high-density and high-strength silicon carbide ceramic materials, various additives have been utilized. For example, a method of hot pressing silicon carbide to densities in order of 98% of theortical by addition of aluminum and iron as densification aids is disclosed by Alliegro, et al., J. Cram. Soc., Vol. 39, No. 11, Nov., 1956, pages 386 to 389. They found that a dense silicon carbide could be produced from a powder mixture containing 1% by weight of aluminum. Their product had a modulus of rupture of 54,000 psi at room temperature and 70,000 psi at 1371.degree. C. More recent advance is the use of boron as a densification additive, usually in the range of between about 0.3 and about 3.0 percent by weight of the powder. The boron additive may be in the form of elemental boron or in the form of boron-containing compounds, for example, boron carbide. Examples of boron-containing silicon carbide powders may be found in U.S. Pat. Nos. 3,852,099; 3,954,483; and 3,968,194.